1. Field of the Invention
The invention relates to a method and apparatus for controlling the real and reactive power behavior of a high voltage D.C. transmission (HDT) system during a line disturbance, and more particularly, to a method and apparatus for adjusting the current and control angle regulators of an HDT system to protect against system transients and overvoltages occurring after a line disturbance.
2. Description of the Prior Art
In prior HDT systems, after tripping of the transmission line protective devices of one station-half, the faulty station-half is generally cut out for a short time. The resultant load drop causes a voltage rise in the three-phase driving network or the rectifier transformer, which endangers the rectifier station connected to the transformer, and in particular the controllable rectifier valves.
This problem is solved by a known method in which, in case attempts to get the faulty pole back on line fail, the regulation of the intact station-halves is timely adjusted in amplitude and phase for the purpose of preventing overvoltage in the driving three-phase network. In particular, in the known method the control angle in the intact rectifier station is increased constantly to a maximum value of about 60.degree. to 70.degree. in about 10 to 25 ms from the onset of the fault (DT-OS No. 2,418,468). The control of the intact rectifier and the inverter station halves by varying of the control angle of the rectifier station-half and the current set-points of the rectifier and inverter station-halves during a momentary fault must correspond to the regulation provided in the normal case. This measure and the maintenance of true power transmission, within the limits of the intact HDT, are not given enough consideration in the known case.
It is further known how to construct an HDT system for the regulation of reactive power or line voltage in addition to the transmission of true power DT-OS No. 1,962,042, U.S. Pat. No. 3,949,291). A disadvantage of this method is a loss in transmission efficiency proportional to the line length. Therefore this regulation method is economical only for short times.
Even though an HDT system of significant transmission length is not normally used for line voltage regulation, this does not preclude an occasional application in special cases. In particular, an HDT system of the above-mentioned type can be used for controlling line overvoltages as occur, e.g., in connection with an HDT load drop, such that the level of isolation of the HDT can be lowered without great expense, thus leading to savings in rectifier construction.
In a known method, for avoiding the overvoltage occurring during a load drop in an HDT system, as disclosed in DT-OS No. 1,943,646 and U.S. Pat. No. 3,801,895, when a line protective device of a faulty station-half is tripped, the D.C. current of the intact station-half is increased by dropping the voltage at the inverter in such a way that the intact station-half increases its rectifier reactive power to the extent that the intact station-half draws from the three-phase network about the same reactive power as was drawn by both station-halves together before the disturbance. In the event of rapid voltage change at the inverter, the rectifiers, constructed from the thyristors at both the converter and the inverter, can be overloaded by the heavier current flow. The control angle is then so diminished, in order to decrease the voltage at the inverter station, that it is shifted in direction at the rectifier drive, but is held constant at the inverter end. In order, under the assumptions, to hold the reactive power reduction of the HDT constant under the aforesaid fault condition, the D.C. current of the remaining station-halves must be increased by a factor less than 2. The value is not doubled because the commutation reactive power increases not only with the value of the D.C. current but also, and quite sharply, with increase in firing angle. In the known case the determination has of course been made as to whether the remaining current overload of the thyristor valves is safe. Decisive to this determination is the permissible peak value of the forward current as a function of overcurrent duration. Assuming an overcurrent duration about equal to the arc de-ionization time, one derives the permissible peak current from the maximum load curve. The thyristors, if necessary, are so overloaded for a conduction period corresponding to the arc de-ionization time, that the above-mentioned constancy of the reactive power reduction is assumed. Along with this optimization between the utilization of the valves and the reactive power output, there is the further desire to impair the true power transmission as little as possible. The above-mentioned reference, U.S. Pat. No. 3,801,895, gives no indication of how this is to be done.